Vehicle emission control systems may be configured to store fuel vapors from fuel tank refueling and diurnal engine operations in a charcoal canister. During a subsequent engine operation, the stored vapors can be purged into the engine where they are combusted. For example, an intake manifold vacuum generated during engine spinning can be used to draw in the stored fuel vapors. As another example, boosted intake air may be directly or indirectly used to purge the fuel vapors to the engine.
During canister purging, an engine control system may attempt to purge the canister as fast as possible, and also as completely as possible. However, various purging limits may be encountered pertaining to the maintenance of an engine air-fuel ratio as well engine air flow rates. As an example, when the canister is nearly full and/or when ambient conditions are hot, the canister purge rate may be limited to be a fraction of the engine's fuel injection rate. In another example, when the canister is partially full or when ambient conditions are cold, and while the engine is at a low load idle condition, the canister purge rate may be limited based on the amount of air that can be ingested by the engine. As such, if the amount of un-throttled air entering the engine becomes too large, fuel consumption may increase to maintain the engine at stoichiometry. As a further example, when the canister is nearly full and/or when ambient conditions are cold, the canister purge rate may be limited by the flow rate of the canister purge valve. The various limitations enable fuel flow from the canister to the engine intake to be rapidly reduced when the engine air flow rate drops, such as due to a closed throttle event.
However, the inventors herein have recognized that the limitations imposed on the canister purge flow rate can result in insufficient canister purging which degrades exhaust emissions. The inventors have realized that higher purge rates may be applied over a wider range of operating conditions by regulating purge flow to be a defined proportion of intake airflow over the operating conditions. For example, during purging conditions when the canister load is higher, such as when the canister is almost full, higher purge rates can be tolerated due to higher air flow to the engine and higher rates of fuel consumption. During conditions when the canister load is lower, such as when the canister is almost empty, higher purge flow rates can be used to more completely empty the canister without incurring over-fueling issues.
As such, conventionally purge fuel flow rates have been the focus of purge control. However, the inventors have recognized that purge control can be improved by focusing on a normalized purge air-fuel ratio (or phi_purge) instead. Focusing on phi_purge allows a higher net purge flow rate over a given purge period. Accordingly, the first limitation changes to phi_purge, not purge fuel flow rate. The second limitation is then the sum of the purge fuel rate and purge air rate that an engine is able to ingest without unintentionally increasing fuel consumption.
Thus in one example, canister purging can be performed more completely over a larger range of engine operating conditions using a method for an engine, comprising: receiving purge flow from a fuel system canister at each of a first location upstream of a compressor and a second location downstream of an intake throttle, and adjusting the purge flow to be a preselected proportion of total engine fuel. In this way, a ratiometric purge may be enabled.
As an example, during purging conditions, a purge flow through a canister may be adjusted so that the purge fuel vapors constitute a fixed preselected portion, such as substantially 20%, of total engine fuel. As such, the fuel vapor fraction of total engine fuel may be maintained as engine fueling changes with engine load from a minimum engine fueling to a maximum engine fueling. Thus, as the total engine fuel increases or decreases, the purge flow may be correspondingly adjusted. A liquid fueling of the engine from a fuel injector may be adjusted to provide the remaining fuel fraction. As such, the purge flow may be also be affected by the canister load. Thus, to maintain the preselected fuel vapor fraction, a lower purge flow rate may be applied when the canister load is higher, while a higher purge flow rate may be applied when the canister load is lower. In addition, as the canister load decreases, the fuel vapor fraction may decrease, and the liquid fuel fraction may be correspondingly increased to maintain the air-fuel ratio.
A location of the purging may also be adjusted based on operating conditions. Specifically, purge flow may be drawn into a first location upstream of an intake compressor (and upstream of an intake throttle) or a second location downstream of an intake throttle (and downstream of the intake compressor). In some embodiments, a portion of the purge flow may be directed to the first location while a remaining portion of the purge flow is directed to the second location. The routing may be based on engine operating conditions including boost pressure and manifold pressure. When manifold pressure is lower (e.g., high engine loads), engine intake vacuum may be used to draw the purge vapors into the intake downstream of the throttle, while maintaining the purge fuel fraction at the determined proportion. When manifold pressure is higher (e.g., low engine loads), compressor bypass flow may be used to drawn vacuum at an aspirator, the aspirator vacuum used to purge fuel vapors at the determined proportion upstream of the compressor.
During engine operating conditions when engine intake airflow is higher, intake airflow may not be limited and total engine fueling requirement may be higher. Thus, a larger absolute amount of purge fuel vapors may be drawn into the engine downstream of the throttle from a highly loaded canister using intake vacuum, albeit at a lower normalized purge fuel air ratio. Thus when the canister load is higher, and while airflow is not limited (such as while boost is being built), a canister purge valve opening may be increased so that the purge fuel vapors can be drawn into the engine intake, at a location downstream of an intake throttle, via a first purge conduit. As the canister load decreases, and while the engine is still operating unboosted, a higher canister airflow rate is applied to maintain the same purge vapor fuel mass rate or normalized purge fuel air ratio. The increased airflow through the mostly empty canister advantageously warms the canister, improving desorption of fuel vapors from the canister and improving complete purging of the canister. During engine operating conditions when engine load is lower and engine intake airflow is lower, intake airflow may be limited, and the engine may operate boosted. During such conditions, the purging may be directed to the location upstream of the compressor by adjusting the position of an ejector shut-off valve to control motive flow and vacuum generation at the ejector. By varying the ejector vacuum, the purge flow to the upstream location can be adjusted so that the preselected proportion of total engine fueling is provided via the purge flow, with the purge flow rate increased as the canister load decreases. Alternatively, the ejector shut off valve may be an open/shut valve. In this case, a canister purge valve is useful for regulating flow into the ejector's suction port.
In this way, as total engine fueling transitions from a maximum engine fueling condition to a minimum engine fueling condition, a purge flow rate from a fuel system canister and a location of receiving the purge flow may be adjusted so that the purge fuel vapors constitute a fixed pre-selected proportion of the total engine fueling. This approach allows the canister to be gradually purged when it is highly loaded and when the engine fueling rate is higher. The approach further allows the canister to be more completely purged when it is lightly loaded and when the engine fueling rate is lower by flowing more air through the canister. By maintaining the purge flow to be a fixed proportion of total engine fueling, even as engine fueling rates change, a higher purge flow rate can be used at the canister, on average. This allows for a more complete purging of the canister, improving canister adsorption efficiency and exhaust emissions. In addition, by enabling higher purge rates, a larger proportion of the engine fuel can be provided as fuel vapors, reducing the liquid fueling required, and providing fuel economy benefits. By using ratiometric purging, the purge fuel mass rate increases proportionally with total engine fuel mass rate up to the point when the fuel vapor storage system hits a plumbing-related flow limitation. In this case, it is the plumbing that limits further purge flow rate, not the classic “maximum allowed purge fuel mass rate” limitation.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.